Base stations used in wireless telecommunication systems have the capability to receive linear polarized electromagnetic signals. These signals are then processed by a receiver at the base station and fed into the telephone network. In practice, the same antenna which receives the signals can also be used to transmit signals. Typically, the transmitted signals are at different frequencies than the received signals.
A wireless telecommunication system suffers from the problem of multi-path fading. Diversity reception is often used to overcome the problem of severe multi-path fading. A diversity technique requires at least two signal paths that carry the same information but have uncorrelated multi-path fadings. Several types of diversity reception are used at base stations in the telecommunications industry including space diversity, direction diversity, polarization diversity, frequency diversity and time diversity. A space diversity system receives signals from different points in space requiring two antennas separated by a significant distance. Polarization diversity uses orthogonal polarization to provide uncorrelated paths.
As is well-known in the art, the sense or direction of linear polarization of an antenna is measured from a fixed axis and can vary, depending upon system requirements. In particular, the sense of polarization can range from vertical polarization (0 degrees) to horizontal polarization (90 degrees). Currently, the most prevalent types of linear polarization used in systems are those which use vertical/horizontal and +45.degree./-45.degree. polarization (slant 45.degree.). However, other angles of polarization can be used. If an antenna receives or transmits signals of two polarizations normally orthogonal, they are also known as dual polarized antennas.
An array of slant 45.degree. polarized radiating elements is constructed using a linear or planar array of crossed dipoles located above a ground plane. A crossed dipole is a pair of dipoles whose centers are co-located and whose axes are orthogonal. The axes of the dipoles are arranged such that they are parallel with the polarization sense required. In other words, the axis of each of the dipoles is positioned at some angle with respect to the vertical axis of the antenna array.
One problem associated with a crossed dipole configuration is the interaction of the electromagnetic field of each crossed dipole with the fields of the other crossed dipoles and the surrounding structures which support, house and feed the crossed dipoles. As is well known in the art, the radiated electromagnetic fields surrounding the dipoles transfer energy to each other. This mutual coupling influences the correlation of the two orthogonally polarized signals. The opposite of coupling is isolation, i.e., coupling of -30 dB is equivalent to 30 dB isolation. Dual polarized antennas have to meet a certain port-to-port isolation specification. The typical port-to-port isolation specification is 30 dB or more. The present invention provides a means to increase the port-to-port isolation of dual polarized antenna systems with a simple parasitic element positioned transverse to a vertical axis of the backplane approximately midway along the length of the backplane. The present invention further provides a means to improve the port-to-port isolation and cross polarization of dual polarized antenna systems with a simple plate, having generally square apertures, that is displaced above a top side of the backplane. In both the parasitic element and the square aperture plate embodiment, the isolation results from the phase-adjusted re-radiated energy that cancels with the dipole mutual coupling energy.
Generally, dual polarized antennas must meet the 30 dB isolation specification in order to be marketable. Not meeting the specification means the system integrator might have to use higher performance filters which cost more and decrease antenna gain. The present invention overcomes these concerns because it meets or exceeds the 30 dB isolation specification.
Another problem with prior antennas is the attachment of the protective radome to the backplane of the antenna. Because of the manner of attachment of prior radomes, prior radome designs have allowed water and other environmental elements to enter the antenna, thereby contributing to corrosion of the antenna. Furthermore, because those prior radomes are loose and not tightly secured to the backplane, such radomes allow the radome to move with respect to the backplane thus allowing wind and water to enter the antenna.
Moreover, the visual impact of base station towers on communities has become a societal concern. It has become desirable to reduce the size of these towers and thereby lessen the visual impact of the towers on the community. The size of the towers can be reduced by using base station towers with fewer antennas. This can be achieved if dual polarized antennas and polarization diversity are used. Such systems replace systems using space diversity which requires pairs of vertically polarized antennas. Some studies indicate that, for urban environments, polarization diversity provides signal quality equivalent to space diversity. With the majority of base station sites located in urban environments, it is likely that dual polarized antennas will be used in place of the conventional pairs of vertically polarized antennas.